WITH

Databend supports common table expressions (CTEs) and allows you to use a WITH clause to define one or multiple named temporary result sets that are used by the query that follows. The "temporary" means that the result sets will be not permanently stored anywhere in the database schema. They act as temporary views that are only available to the query that follows.

When a query with a WITH clause is executed, the CTEs within the WITH clause are evaluated and executed first. This produces one or multiple temporary result sets. Then the query is executed using the temporary result sets that were produced by the WITH clause.

This is a simple demonstration that helps you understand how CTEs work in a query: The WITH clause defines a CTE and produces a result set that holds all customers who are from the Québec province. The main query filters the customers who live in the Montréal region from the ones in the Québec province.

WITH customers_in_quebec 
     AS (SELECT customername, 
                city 
         FROM   customers 
         WHERE  province = 'Québec') 
SELECT customername 
FROM   customers_in_quebec
WHERE  city = 'Montréal' 
ORDER  BY customername; 

CTEs simplify complex queries that use subqueries and make your code easier to read and maintain. The preceding example would be like this without using a CTE:

SELECT customername 
FROM   (SELECT customername, 
               city 
        FROM   customers 
        WHERE  province = 'Québec') 
WHERE  city = 'Montréal' 
ORDER  BY customername; 

Syntax

WITH cte_name1 [(col_name [, col_name] ...)] AS (subquery1)
[, cte_name2 [(col_name [, col_name] ...)] AS (subquery2)]
[...]
SELECT ...

Where:

WITH: Initiates the WITH clause.

cte_name1: Specifies the name of the first result set.

subquery1: Defines the first result set.

cte_name2 AS (subquery2): You can define multiple CTEs in a WITH clause.

A CTE can refer to any CTEs in the same WITH clause that are defined before.
When you have multiple CTEs, separate them with commas.

SELECT ...: CTEs are mainly used with the SELECT statement.

Examples

Imagine you manage several bookstores located in different regions of the GTA area, and use a table to hold their store IDs, regions, and the trading volume for the last month.

CREATE TABLE sales 
  ( 
     storeid INTEGER, 
     region  TEXT, 
     amount  INTEGER 
  ); 

INSERT INTO sales VALUES (1, 'North York', 12800);
INSERT INTO sales VALUES (2, 'Downtown', 28400);
INSERT INTO sales VALUES (3, 'Markham', 6720);
INSERT INTO sales VALUES (4, 'Mississauga', 4990);
INSERT INTO sales VALUES (5, 'Downtown', 5670);
INSERT INTO sales VALUES (6, 'Markham', 4350);
INSERT INTO sales VALUES (7, 'North York', 2490);

The following code returns the stores with a trading volume lower than the average:

-- Define a WITH clause including one CTE
WITH avg_all 
     AS (SELECT Avg(amount) AVG_SALES 
         FROM   sales) 
SELECT * 
FROM   sales, 
       avg_all 
WHERE  sales.amount < avg_sales;

Output:

Markham|6720|9345.71428571429
Mississauga|4990|9345.71428571429
Downtown|5670|9345.71428571429
Markham|4350|9345.71428571429
North York|2490|9345.71428571429

The following code returns the average and total volume of each region:

-- Define a WITH clause including two CTEs
WITH avg_by_region 
     AS (SELECT region, 
                Avg (amount) avg_by_region_value 
         FROM   sales 
         GROUP  BY region), 
     sum_by_region 
     AS (SELECT region, 
                Sum(amount) sum_by_region_value 
         FROM   sales 
         GROUP  BY region) 
SELECT avg_by_region.region, 
       avg_by_region_value, 
       sum_by_region_value 
FROM   avg_by_region, 
       sum_by_region 
WHERE  avg_by_region.region = sum_by_region.region; 

Output:

Downtown|17035.0|34070
Markham|5535.0|11070
Mississauga|4990.0|4990
North York|7645.0|15290

Syntax​

Examples​

Syntax

Examples